1. Field of the Invention
The present invention relates to a flow control valve adapted to maintain a minimum flow rate by bypassing fluid for the purpose of preventing overheating or other hazards of a pump when a flow rate is small.
2. Description of the Prior Art
One example of a flow control valve in the prior art is shown in FIG. 5. A valve box 10 opens at an inlet 1i, an outlet lo and a bypass outlet 1b. Within the valve box 10 is provided a main valve element 20 pressed against a main valve seat 1s by means of a spring 30, and a bypass opening/closing body 50 connected to the main valve element 20 is supported in an axially movable manner. In the bypass opening/closing body 50 are formed a hollow space portion 5a and an elongated hole 5c. When a demand flow rate on a load side is large, the pressure of fluid entering through the inlet 1i into the valve box 10 pushes the main valve element 20 rightwards as viewed in FIG. 5 from the main valve seat 1s to open the main valve element 20, and thereby the fluid flows out from the outlet 1o to the load through a gap clearance formed around the main valve body 20. At this time, the bypass opening/closing body 50 moves rightwards as viewed in FIG. 5 jointly with the main valve element 20, and the elongated hole 5c is held at a position where it comes into contact with an inner surface of a guide member 19. Hence, the fluid entering into the hollow space portion 5a has no path for communicating with the bypass outlet 1b, and the bypass flow rate is nearly zero. If the load flow rate has reduced, since the main valve element 20 approaches to the main valve seat 1s, being pushed leftwards in FIG. 5 by the spring 30, the bypass opening/closing body 50 also moves leftwards jointly with the main valve element 20, and the elongated hole 5c comes to the position where it communicates with the bypass outlet 1b. The fluid entering through the inlet 1i then flows from the hollow space portion 5a through the elongated hole 5c and escapes from the bypass outlet 1b. This bypass flow rate is preset as shown in FIG. 6(B) so that a minimum flow rate can be maintained for the purpose of preventing overheating or other hazards of a pump when the flow rate is small.
However, in the case of the above-described flow control valve in the prior art, depending upon the position of the main valve element 20, the position of the bypass opening/closing body 50 is determined, and hence the bypass flow rate is also definitely decided. Accordingly, in the event that it is desired to change the bypass start point and the like after this heretofore known flow control valve has been assembled in a plant or the like, it is impossible to change and adjust it unless the control valve is disassembled. In addition, in the case where the fluid is high-pressure hot water, there is a problem in that due to a large pressure difference of the pressure reduction at the elongated hole 5c, cavitations arise, erosion is generated in the proximity of the elongated hole 5a, resulting in a change of the shape of the elongated hole 5c, and therefore bypass characteristics change.